When astronauts return to the moon by 2020, they will do so following a familiar path and flying in a cone-shaped capsule that echoes the "good old days" of Project Apollo.
But the skin-deep similarities between Apollo's Command Module and Constellation's Orion spacecraft are far outnumbered by differences — more like improvements — that NASA's 50 years of spaceflight experience make possible.
"We've learned a lot, but we still have a lot of questions," said Bret Drake, Chief Architect in the System Engineering and Integration Office for Constellation at the Johnson Space Center in Houston.
For example, the best Apollo could offer by 1972's final lunar excursion was a two-man crew lingering for about three days at a landing site that was generally near the moon's equator.
Constellation is shooting for a means to land anywhere on the moon and eventually grow the capability to build up a permanent presence at one site, likely near one of the poles, Drake said.
"We're trying to evolve a capability, right out of the box, where we're going to have four-member crews staying on the surface for seven days," Drake said. "But eventually we'll get to the point where we have four crewmembers on the surface for 180 days."
That potential still is more than a decade away, and though the fine details of what all the hardware is going to look like continue to evolve, the picture of how the United States will go back to the moon is coming into focus.
"Functionally, physics drives you in a lot of the same directions we went through in Apollo," Drake said. "We're doing a lot more with the systems than we were with Apollo."
Skin deep beauty
To start, that means the Orion spacecraft will be shaped like a cone, Drake said.
When determining the best shape to deal with the speeds and heat of reentry encountered by a spacecraft returning from the moon, the laws of physics are the same in the 21st Century as they were in the 20th Century.
So from a distance the Orion spacecraft looks almost identical to the Apollo Command Module. Up close the differences become more apparent, especially the size.
Orion is now set to be 16.5 feet (5 meters) in diameter and 10.8 feet (3.3 meters). It will weigh about 31,000 pounds (14,000 kg) empty and have a habitable volume of 692 cubic feet (11 cubic meters).
Apollo's Command Module by comparison: A diameter of 12.8 feet (3.9 meters) and a height of 11.4 feet (3.47 meters). Total dry weight was 12,787 pounds (5,800 kg) and its crew cabin volume was 218 cubic feet (6.17 cubic meters).
Both spacecraft had or will have an attached Service Module that will have a single large engine fed by storable propellants and contain storage and bays to support the main cabin systems.
Both Apollo and Orion's Service Module will have batteries and electricity generating fuel cells, but the Orion also has a pair of solar arrays to help keep the vehicle powered during its intended long stay in lunar orbit.
And that brings to light some of the key difference between the Apollo and Orion mission scenarios, or more simply, the paths they will follow from the Earth to the Moon.
From the Earth to the moon
During Apollo, a single three-stage Saturn V Moon rocket carried the crew, Command Service Module and the Lunar Module into Earth orbit. After one orbit the Saturn V's third stage was re-ignited and sent Apollo toward the Moon.
After discarding the third stage on the way to the Moon, the Apollo spacecraft entered lunar orbit. At the appropriate time, two of the three crewmen boarded the Lunar Module and went down to the surface, leaving the third crewman in the Command Module.
The plan for Constellation, as it stands now, is to launch a four-person crew in an Orion spacecraft atop an Ares I launch vehicle from the Kennedy Space Center into Earth orbit, where it will wait up to four days, Drake said.
Ninety minutes after the Ares I launch, a heavy-lift Ares V rocket is to launch the Altair lunar landing spacecraft and an Earth Departure Stage into Earth orbit for a same day rendezvous and docking with the Orion capsule.
The joined spacecraft will remain in Earth orbit for up to four days while waiting for the Moon to orbit into the right position when the launch window for a Trans-Lunar Injection opens up. Then the combined spacecraft will blast their way toward the Moon.
After discarding the Earth Departure Stage, the Altair lunar lander's rocket engine — powered by liquid hydrogen and liquid oxygen — will brake the spacecraft into orbit around the moon.
This is different from Apollo, which relied on the Service Module's main service propulsion system engine for the same job.
Location is everything
Although plans for the first mission or so may change, Drake said the baseline plan is for all four astronauts to board Altair for the trip to the lunar surface, leaving an empty Orion spacecraft to circle the moon on its own.
That's a big difference from Apollo, which left one astronaut in the Command Module to keep that spacecraft healthy and the crewmember busy with "bird's eye" science observations of the surface.
As for the selection of a lunar landing site, during Apollo a variety of sites were explored, all generally near the moon's equator. For Constellation, a single site will be selected, likely near one of the poles.
Among the reasons, according to Drake:
With each landing NASA will leave equipment there that can be joined with other hardware to build up a permanent station on the surface. It may be possible to establish this site in a place on the Moon where there is always sunlight, which can help with generating electricity and maintaining the correct thermal conditions with the surface hardware. It is also possible that a crater near the poles that remains in constant darkness may hide water ice, which can be broken down into hydrogen and oxygen for use in powering fuel cells, producing rocket propellant and making drinking water.
Under the hood
Systems inside Apollo versus Orion also will be very different.
While Apollo had a switch for everything, Orion will have glass cockpit displays and touch-sensitive screens that can be programmed on the fly.
While Apollo relied on computers with less power and memory than a digital toy found inside a cereal box, Orion will have the latest computers plugged into an open architecture that allows upgrades to software and hardware.
While Apollo used a pure oxygen atmosphere in space, Orion will use a nitrogen-oxygen mixture at reduced pressure to make it easier for spacewalking or Moonwalking astronauts to depart the spacecraft.
And while Orion's heat shield will use an ablative material and parachutes for a splashdown in the water like Apollo, unlike Apollo there continues to be talk about how much of an Orion spacecraft can be reused after flying, Drake said.
"We have a lot of options on the table," Drake said of the entire Orion system.